How can I use an image texture as a guide for weight painting?
Here is my solution:
Create a plane and subdivide it (as many times as you like, depending on your weight paint resolution requirements). Unwrap the mesh. Now you want to create a sphere from a plane using shapekeys. See the Cegaton's answer on how to do it here: What is the best way to unwrap a sphere?


When your transformation is ready, check the Basis key and add your heightmap image as a background image. Scale the added image to fit the size of a plane and check the image display type to Front. Set the Opacity value as you like.


Now you can easily paint in Weight Paint Mode, seeing a heightmap texture.


Transform a plane to a sphere using shapekeys.


As you can see all work quite nice.